PROJECT SUMMARY/ABSTRACT We propose to study the relationship of neuroimaging abnormalities and neuropathology in mucopolysaccharidosis I (MPS I), a lysosomal storage disease that strikes in infancy or childhood. Children with MPS I develop hydrocephalus, atrophy, cystic or cribriform changes, and white matter abnormalities including decreased volume and fractional anisotropy of the corpus callosum (a white matter structure). The cause of hydrocephalus is thought to be decreased reabsorption of cerebrospinal fluid from storage in the arachnoid granulations, and the cause of cystic or cribriform lesions is probably the accumulation of glycosaminoglycans in perivascular (Virchow-Robin) spaces. However, the underlying basis of atrophy, white matter hyperintensities, and reduced volume and fractional anisotropy in the corpus callosum (and probably other white matter structures) is not known. The basis of these latter findings is important to understand, because they have been found to correlate with cognitive impairment in MPS patients. We hypothesize that the underlying basis of white matter abnormalities on imaging studies is dysmyelination. We have found evidence of significant dysmyelination in the corpus callosum of an MPS I animal model which is consistent with the reduced volume and fractional anisotropy in that white matter structure. The hypothesis that dysmyelination underlies these imaging findings will be directly tested, and we will also test alternate hypotheses that gray matter disease or hydrocephalus is responsible. Methods employed will include high-resolution magnetic resonance imaging, volumetrics, diffusion tensor imaging, ultrastructural studies of white and gray matter, evaluations of myelin components, and other evaluations of gray and white matter pathology. Interventions tested will include treatment with intrathecal enzyme replacement therapy in the juvenile period and ventriculoperitoneal shunting for hydrocephalus. The results will lead to a new, central model to connect neuropathology and neuroimaging findings, which are the key to understanding the pathogenesis of MPS- related brain disease.